There are numerous efforts underway to generate hydrocarbon base oils from renewable biomass. (e.g., Xiong et al., A Bio-Catalytic Approach to Aliphatic Ketones, Science Reports, 2, 311, pp. 1-7, Mar. 13, 2012). One approach is to generate aliphatic ketones from fatty acids or their derivatives. The aliphatic ketone or mixture of aliphatic ketones produced can then be deoxygenated to give saturated hydrocarbons.
The preparation of ketones from carboxylic acids is well known and has been used in the preparation of ketones from carboxylic acids for many years. (e.g., Vogel's Textbook of Practical Organic Chemistry, Forth edition, Longman N.Y. 1978, pp. 429-433). It is also well known that the reactions can be used to convert fatty acids such as stearic acid and other fatty acids of natural origin to ketones in the base oil boiling range. (e.g., U.S. patent application Ser. No. 13/157,921; and U.S. Pat. Nos. 7,850,841; 7,967,973; and 8,048,290). Conventional ketonization operates at high temperatures (e.g., ≧300° C.) and can produce significant amounts of by-products such as light fragments formed in various ways such as by cracking of the fatty acid chains or by decarboxylation of the fatty acid. The formation of these by-products limits the overall selectivity and thus the yield of the desired ketone product. There has so far been a limited understanding of the factors that causes the relatively poor selectivity. In addition, the side reactions that cause the lower selectivity also tend to cause faster catalyst passivation.
We have discovered that the selectivity in the reaction of fatty acids to form ketones is dependent on the partial pressure of the fatty acids. Intuitively one might have expected that a condensation reaction such as the reaction of two molecules of fatty acid to form one molecule of ketone would be favored by higher partial pressure of the reactants, but to our surprise we have observed that the opposite is the case. We have found that the ketone selectivity and the catalyst stability is favored by low partial pressure of the fatty acids. In fact, it proves to be of critical importance to the selectivity that the partial pressure of the fatty acid in the reactor is kept below the pressure at which the fatty acid could condense into a liquid phase at the reaction temperature.